Gasification of carbonaceous materials typically involves a thermal reaction between the carbonaceous material, oxygen and steam to create a mixture of low weight hydrocarbons, such as methane, carbon monoxide and hydrogen (syngas). Gasification is widely used to produce syngas for firing or syngas for refining into chemicals, liquid fuels and hydrogen, and has been identified as a key enabling technology for advanced high-efficiency, low-emission non-fossil fuel and renewable energy power generation.
High temperature gasification and other medium to light combustion air input thermal processes generate turbulent hot gases. In turn, the turbulent hot gases facilitate pneumatic agitation of the contents of a furnace, thus assisting in consumption of the carbonaceous material as “fresh” surfaces are brought into contact with the process reactants.
Pneumatic agitation, however, can result in entrainment of solids, heavy metals and ash in the resulting syngas product stream, which is then treated by downstream filtration techniques and/or scrubbing to remove the entrained solids and ash.
The application of high temperature gasification and other medium to high combustion air input thermal processes to manage municipal waste presents many difficulties, particularly because of the lack of homogeneity of the contents in terms of size and composition compared to other carbonaceous materials such as coal and blomass.
The average moisture content of municipal waste may vary from 20-60%, or higher, and the average incombustible content may vary from 5-30% or higher, with some waste charges having 100% incombustible items (e.g., glass, metals, etc.). A high incombustible content results in a high density charge with concomitant increased accumulation of incombustibles/ash content The larger percentage of inorganic solids and ash that is not consumed by combustion processes leads to an increase in the downstream clean-up processes required to provide a syngas product stream and reduced production efficiency.
Further, the high incombustible or ash content accumulates in the gasification or combustion chamber and depletes the available space in the gasification or combustion chamber. After 6-8 hours of operation under typical conditions, several issues can occur if the volume occupied by incombustible material and ash is not reduced by ejection, including:                1) loss of control of process conditions (e.g., temperature, throughput velocities, etc.);        2) increased process gas velocity/turbulence leading to increased particulate entrainment in the syngas product stream;        3) incoming raw waste compacts the incompletely gasified or combusted material and ash in the combustion chamber, causing further reduced thermal efficiency; and        4) compaction of raw waste leads to obstructions in the feed stream.        
Furthermore, complete gasification or combustion of the contents is not always achievable as pneumatic agitation may not prove sufficiently strong to bring larger, heavier particles in the waste solids in contact with the combustion reactants. Waste that has a high moisture content, incombustible content and density can self insulate from the gasification or combustion process and form sections or “pockets” of coagulated or partially degraded waste that substantially reduces thermal efficiency, in addition to partially degraded matter being ejected with ash.
In contrast, low temperature gasification relies on thermal degradation of the carbonaceous material in an oxygen-depleted ultra-low sub-stoichiometric environment, rather than combustion reactions, to produce a syngas product stream.
The application of low temperature gasification in the management of heterogeneous mixtures of municipal waste minimizes the problem of entrained solids and ash in the syngas product stream because there is little or no pneumatic agitation of the contents of the furnace in which low temperature gasification occurs. Conversely, however, in the absence of pneumatic agitation, there is little mixing of the waste solids within the furnace, resulting in the stratification of the contents where thermally degraded material overlies unreacted carbonaceous material. Where inorganic material, particularly silica-containing material, thermally degrades to form a slag, a mechanical barrier to further thermal degradation of the underlying contents may form, and many of the problems listed above may also arise.